1. Field of the Invention
The present invention relates to a process for producing microcapsules. In greater detail, it relates to a process for producing microcapsules in which a hydrophobic core material is covered with melamine-formaldehyde resin in the aqueous medium.
2. Description of the Prior Art
Microencapsulation is carried out to change the apparent state and properties of the core material, protect the material in a finely-divided form, control release, and release the contents at the time desired.
In recent years, application of microcapsules to image recording materials, medicines, perfumes, agricultural chemicals, chemicals, adhesives, foods, detergents, dyes, solvents, catalysts, enzymes, rust inhibitors, etc., has been studied, and pressure-sensitive copying paper, aspirin capsules, perfume capsules, pressure-sensitive capsule adhesives, active charcoal capsules, enzyme capsules, liquid crystal capsules and methylparathion capsules, etc. have been put to practical use.
Further, for the purpose of satisfying certain functional, operational and economic aspects of production, various encapsulation processes have been proposed. As generally known processes for producing microcapsules, there are physical processes, mechanical processes, physiochemical processes and chemical processes. However, the physical processes and the mechanical processes are only utilized for production of capsules having special uses, because they require a particular apparatus. The resulting capsules are large particles having a particle size of several ten microns or more and the tightness of the membrane of capsule wall is insufficient.
Physiochemical and chemical processes have the advantages that they do not require a special apparatus, it is possible to produce capsules having any desired particle size ranging from less than 1 micron to several millimeters, and it is possible to control the tightness of the membrane of capsule wall. Accordingly, they are of great practical value, because they can be used for various purposes. Examples of these processes include coacervation, interfacial polymerization and in situ polymerization. The coacervation process has been used in a wide variety of fields, but it has the drawbacks that the capsules produced have inferior water resistance, they are expensive, and a capsule solution having a high concentration is difficult to obtain because gelatin is an indispensable component and the steps of encapsulation are complicated. The interfacial polymerization process of forming capsules by a polymerization reaction of a hydrophobic monomer and a hydrophilic monomer on the interface of a core material has the drawbacks that the process involves restricted handling on toxicity, stability, etc., it deteriorates a core material having active hydrogen atoms or encapsulation is impossible, the reaction is difficult to control, and the membrane of the capsule walls is difficult to thicken, etc., because it employs substances having a high reactivity (e.g., polyisocyanates, acid chlorides or epoxy compounds, etc.) as the hydrophobic monomer.
In situ polymerization includes processes wherein the capsule wall membrane is formed from the inside of the core material by polymerization of monomers and wherein the capsule wall membrane is formed from the outside of the core material. The former process has the drawbacks that suitable core materials are limited because polyisocyanates and the like are necessary to obtain good capsule wall membranes. In the latter process, amino resins are generally used for the membrane (for example, urea-formaldehyde and melamine-formaldehyde resins).
In recent years, with the increasing application of microcapsules, processes have been desired in which (1) it is possible to employ a variety of core materials, (2) it is possible to carry out encapsulation at a high concentration and high yield, (3) the cost of encapsulation is low, (4) the encapsulation step can be easily controlled, (5) the capsule wall is durable to temperature, humidity and various solvents (6) the capsule wall does not deteriorate, (7) capsules having a desired particle size and physical strength can be obtained, (8) the capsule slurry has a low viscosity, and (9) the time for encapsulation is short.
The interfacial polymerization process and the in situ polymerization process satisfy the above-described requirements to some degree. However, interfacial polymerization and in situ polymerization in which the capsule wall membrane is formed from the inside of the core material by polymerization of monomers have the drawback that suitable core materials are restricted because a compound having high reactivity is used as the monomer for the capsule wall. Thus, in the in situ polymerization, it is preferred to utilize a process wherein the capsule wall membrane is formed from the outside of the core material by polymerization of a monomer such as described in Japanese Patent Publication Nos. 12380/62, 12518/63 and 14379/69, British Pat. Nos. 1,355,124 and 2,006,709, Japanese Patent Application (OPI) No. 144383/76 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), and U.S. Pat. Nos. 3,516,941, 4,001,140, 4,105,823, 4,089,802, 4,087,376 and 4,100,103, in which urea-formaldehyde resin or melamine-formaldehyde resin is used as a capsule wall.
By comparison microcapsules having a melamine-formaldehyde resin membrane are superior to those having a urea-formaldehyde resin membrane, because the membrane is more resistant to temperature, humidity and various solvents. However, in the production of microcapsules having a melamine-formaldehyde resin membrane, an aggregation of the capsule particles or an increase in the viscosity of the capsule solution easily occur during the encapsulation reaction. As a result, the production of capsules having a melamine-formaldehyde resin membrane must be carried out at low concentrations. Further, because low reaction temperatures are necessary, there is the drawback that the reaction time is long.
Processes which obviate these drawbacks to some degree have been described in U.S. Pat. No. 4,100,103 and British Pat. No. 2,006,709. According to these processes, capsule particle aggregation and increasing viscosity, etc., are overcome by using a copolymer of maleic acid anhydride and an ethylenically unsaturated monomer or a polyacrylic acid as a dispersing agent. However, these processes still do not completely prevent aggregation or the increase of viscosity. Furthermore, ethylene-maleic acid anhydride copolymer (EMA31 produced by Monsanto Co.) used in Example 1 of U.S. Pat. No. 4,100,103 and styrene-maleic acid anhydride copolymer (Scripset 520 produced by Monsanto Co.) used in Example 1 of British Pat. No. 2,006,709 have the drawback that they require a long time for dissolution. Further, the above-described Scripset 520 has the restriction that the pH of the system is difficult to reduce during or after the reaction, because addition of acid causes precipitation and aggregation of the capsules. Consequently, it is difficult to achieve a pH of 4.5 to 2.0 which is effective for removing residual formalin by the addition of urea after the capsulation reaction. Further, it is difficult to add acid to the capsule solution during the reaction to reduce the pH of the system in order to increase the reaction rate or strengthen the membrane.